This invention relates to the field of spiral-type hollow fiber membrane fabric-containing cartridges and modules for separations and other phase contact applications. In particular, this invention relates to an improved cartridge or module having tube sheets which are fabricated from solvent-resistant thermoplastic resins. The teachings of this invention can, if desired, be used in conjunction with the subject matter of applicant's copending U.S. Pat. application Ser. No.: 07/816,511 entitled, "Spiral-Wound Hollow Fiber Membrane Fabric Cartridges and Modules Having Flow-Directing Baffles"; and copending U.S. Pat. application Ser. No.: 07/917,690 entitled, "Spiral-Wound Hollow Fiber Membrane Fabric Cartridges and Modules Having Integral Turbulence Promoters", which are both hereby incorporated by reference in their entirety.
There is a great deal of prior art relating to the structure, fabrication and use of spiral-type hollow fiber-containing cartridges. Among the early disclosures of such devices are Mahon U.S. Pat. No. 3,228,877 and McLain U.S. Pat. No. 3,422,008, both of which are incorporated herein by reference in their entirety. In general, a bundle of hollow fibers is positioned around and parallel to the longitudinal axis of a rod-shaped core (which may be a hollow mandrel and may or may not be removed after the bundle is fabricated), the ends of the hollow fiber bundle are potted in tube sheets, and the cartridge is fitted into a pressure housing suitably ported to facilitate feed, permeate and concentrate flows, to constitute a complete module.
More recently, the art has improved such cartridges by forming the hollow fibers into a fabric-like web in which the hollow fibers, parallel to the core axis, are held in place relative to each other by transverse filaments which may or may not also be hollow fibers. The development of these hollow fiber membrane fabrics was an important step in the evolution of this technology, because the fabric both makes unnecessary the manual handling of a random bundle of loose hollow fibers, and facilitates the construction of spiral-wound modules having the hollow fibers spaced and oriented in a highly regular fashion.
Despite these developments, the challenge to optimize the operability, efficiency and durability of these hollow fiber-containing cartridges has continued. With the development of hollow fiber membrane fabrics, the limiting factor in building a module becomes the fabrication of the tube sheets. The classic hollow fiber potting processes include gravity potting and centrifugal potting. In gravity potting, a resinous potting material is introduced into each of the bundle ends, one at a time, and allowed to settle into the end of the bundle and cure. In centrifugal potting, the bundle is inserted into the housing, the assembly is spun on its midpoint to create centrifugal force at both bundle ends, resinous potting material is introduced into the shell-side space near both bundle ends, and the resinous potting material is allowed to cure. (See generally, Mahon U.S. Pat. No. 3,228,877; Maxwell et al U.S. Pat. No. 3,339,341; Geary et al U.S. Pat. No. 3,442,002; Davis et al U.S. Pat. No. 3,962,094; Opersteny et al U.S. Pat. No. 4,369,605; and Chu et al U.S. Pat. No. 4,865,735). Among the many drawbacks of these procedures are the following: (1) these are labor-intensive manual processes; (2) being manual processes, these procedures do not yield modules of uniformly high quality; (3) the potting resins tend to wick along the dry portions of the hollow fibers, especially in the dipping process; (4) the potting resins must have a sufficiently low viscosity to readily flow among the closely spaced hollow fibers and wet all adjacent hollow fiber surfaces, especially in the case of the centrifugal process; (5) the inherent requirement in these procedures that the potting resin must readily flow among the hollow fibers limits the acceptable potting resins in terms of both solvent resistance and mechanical durability after cure; (6 ) whenever the curing reaction in the potting resin is exothermic, heat buildup within the tube sheet area, particularly the center of the tube sheet, can lead to catastrophic meltdown of the adjacent hollow fiber ends, markedly reducing the operative portion of the bundle or even rendering the module useless (attempts to conventionally pot a cartridge having a diameter greater than about four inches will so fail, unless a resin such as an epoxy or polyurethane resin having low solvent resistance, or a resin including fillers (to conduct heat and/or act as a diluent, reducing the proportion of the total potting resin composition which must undergo exothermic cure), is used; the low exotherm in curing such a resin results in poor curing quality and accordingly poor solvent resistance and/or mechanical properties); (7) gaps or air bubbles in the potting resin due to inadequate flow may cause leaks in the cartridge; (8) the materials conventionally used as potting resins and having good solvent resistance tend to be brittle (e.g., low molecular weight polyethylene); (9) the materials conventionally used as potting resins and being rubbery tend to have poor solvent resistance (e.g., polyurethane); and (10) both the centrifugal and gravity potting techniques necessitate at least one shell-side port on the side of the module housing, into which the potting resin is poured. If such ports are to be used in later module operation, multiple modules cannot be connected in series without exterior piping or specially-designed pressure housings.
Poor tube sheet solvent resistance and mechanical durability can have great impact on the performance features of the resulting modules. The feasible operating environments of the hollow fiber-fabric --containing cartridges and modules ideally would include all types of solvents. No hollow fiber--forming material is inert to all solvents, but conventionally-available hollow-fiber spinning technology does make available a wide range of hollow fiber types. Assuming that one has chosen an appropriate type of hollow fiber for a particular application or range of applications, and formed a fabric from the hollow fibers, poor solvent resistance in the potting resin can result in failure of the cartridge or module, thus limiting the capability of the hollow fibers themselves. Similarly, the purpose of the tube sheets is to isolate the lumen-side and shell-side portions of the cartridge or module from each other. If the tube sheet material cracks or otherwise disintegrates, total failure of the module occurs. Unfortunately, the conventional methods of tube sheet production (centrifugal casting and gravity potting), require a high degree of potting resin flowability, thus limiting the range of usable potting resins in terms of molecular weight, viscosity and other properties and encouraging the development of solvent-induced and mechanical degradation.
Some attempts have been made to overcome one or more of these difficulties by depositing potting material to form tube sheets simultaneously while the hollow fiber bundle is being formed. For example, the Mahon et al U.S. Pat. No. 3,755,034 discloses a process in which (1) two monofilaments are continuously unwound in parallel spaced-apart fashion from spools and onto a mandrel; (2) one or more continuous hollow fibers are continuously unwound transversely across and around the two filaments, forming a planar web of hollow fibers which nearly are mutually parallel and nearly are parallel to the longitudinal axis of the mandrel; (3) the web is wound spirally onto the mandrel; and (4) a band of solidifiable resin is applied adjacent to one or each end of the hollow fibers (near the filament) and subsequently cured to form a resinous tube sheet. Epoxy resins and silicone rubbers are suggested for forming the tube sheets; and the use of hypodermic needles as potting resin dispensing heads is disclosed. The Schrader U.S. Pat. No. 3,728,425 takes a different approach: (1) a single continuous hollow fiber is wound transversely around an elongated cylindrical mandrel core analogously to a bundle of kite string; (2) a band of resin is simultaneously applied at each end of the core, forming tube sheets, and is cured. Polyepoxide resin is disclosed for use, and Schrader employs applicators as shown in FIG. 2 to apply the bands of resin. The Krueger et al U.S. Pat. No. 5,059,374 discloses that the tube sheets may be formed about the ends of a hollow fiber bundle simultaneously with laying down of the fibers, such as by dripping resin along the fibers as the fibers are laid down. Krueger mentions a number of materials that can serve as tube sheet resins, including polyolefins and polyamides. See also, Sargent et al U.S. Pat. No. 3,722,695; Francisoud et al U.S. Pat. No. 4,343,668; applicant's copending U.S. Pat. application Ser. No.: 07/816,511 entitled, "Spiral-Wound Hollow Fiber Membrane Fabric Cartridges and Modules Having Flow-Directing Baffles"; and applicant's copending U.S. Pat. application Ser. No.: 07/917,690 entitled, "Spiral-Wound Hollow Fiber Membrane Fabric Cartridges and Modules Having Integral Turbulence Promoters".
Other prior art disclosures broadly suggesting the use of thermoplastics including polyolefins as tube sheet resins have also been made. For example, the Tigner U.S. Pat. No. 4,138,460 asserts that the choice of material for potting fiber tows to make tube sheets is controlled to a large extent by the viscosity characteristics of the selected material, which should preferably have a viscosity in the range of about 100 to about 5,000 centipoise at a temperature below about 150.degree. C., and is preferably a thermoplastic. Polyethylenes, polypropylenes, and copolymers and mixtures thereof can be used; and reference is made to compositions using low molecular weight polyethylene resins (See a similar disclosure in Lipps et al U.S. Pat. No. 4,211,597 and Lipps et al U.S. Pat. No. 4,231,871, which further note that low viscosity resins tend to completely wet and encapsulate the hollow fibers in shorter times and are therefore preferred). The Brauer et al, U.S. Pat. Reissue No. 31,389 reviews certain materials previously used to fabricate tube sheets, and notes that many of them, including polyolefins and olefin copolymers, have been found deficient in one aspect or another. (See also, Maxwell et al U.S. Pat. No. 3,339,341; Davis et al U.S. Pat. No. 3,962,094; Zampini U.S. Pat. No. 4,323,453; Fritzsche et al U.S. Pat. No. 4,323,454; Otstot et al U.S. Pat. No. 4,686,039; Opersteny et al U.S. Pat. No. 4,369,605; and Chu et al U.S. Pat. No. 4,865,735).
The need continues for improved methods and materials for making tube sheets which will more closely approach the solvent resistance and mechanical durability of the hollow fibers themselves.